When oils (used herein to refer to oils, fats, shortenings and mixtures thereof) are heated in the presence of oxygen and water, thermolytic and oxidative reactions take place, resulting in the degradation of the oils. Most edible oils are triglycerides formed from the reaction of fatty acids and glycerol. Oils are generally considered non-polar in their pure form. However, some oils contain from about 0.5% to about 2% free fatty acids. The presence of these fatty acids may impart some polarity to the fresh oils. However, as these oils are heated at high temperatures (for example, under conditions of deep-frying or under high shear and pressure conditions as can be found for some synthetic oils), the oils can oxidize, polymerize, and/or hydrolyze. Oxidation can generate new functional groups in the hydrocarbon chain of the triglyceride and hydrolysis can generate free fatty acids, monoglycerides and diglycerides. These processes can increase the polarity of the oils.
A number of methods exist in the literature to detect degraded products in oils. Most of the methods are specific to a particular degradation pathway or assay for a particular chemical change. For instance, the measurement of free fatty acids in oil can be used to estimate the level of degradation of oils by the hydrolysis pathway. This method does not quantify the total polar compounds in the oils. Other methods to assess the degradation of oils include, but are not limited to, the determination of hydroxyl number, iodine value, carbonyl value, decreases in unsaturation, smoke point and viscosity changes. There are commercial devices to detect one or more changes in oil that occur with use and each of these methods has its own drawbacks. For example, the FOODOIL-SENSOR (Northern States Instrument, Circle Pines, Minn.) monitors the change in the dielectric constant of frying oils but this instrument must be calibrated to fresh oil daily. Because different oils have different dielectric constants, separate information must be provided for each type of oil to determine whether or not the change in the dielectric constant of a particular oil indicates the oil should be discarded. These types of instruments may not give consistent results over time because the test is also sensitive to environmental factors, such as air drifting, or sensitive to the presence of food particles in the oil itself. The instruments do not operate well in a draft, such as occur in locations near ventilating systems that circulate air. Many deep-fat fryers are positioned under or close to strong air uptake currents making the test ill-suited for use in some restaurants. Libra Labs (Metuchen, N.J.) and U.S. Pat. No. 4,349,353 to Blumenthal disclose tests to detect the presence of alkaline compounds that accumulate in used frying oils. U.S. Pat. No. 4,623,638 to Hayatsu et al. discloses a silica gel that absorbs and desorbs polycyclic organic substances in a solution to detect and remove mutagenic substances from the environment and foodstuffs.
Colorimetric tests based on the pH of the oil are available. For example, the OXIFIT test (E. Merck, Darmstadt, Germany) is a colorimetric test kit that contains redox indicators that react with the total amount of oxidized compounds in the oil. The 3M Shortening Monitor (Minnesota Mining and Manufacturing, St. Paul, Minn.) is a paper strip containing base as an indicator that changes color when the base reacts with fatty acids in the oil. A FRITEST (E. Merck) is available and is a colorimetric test kit that is sensitive to carbonyl compounds. Robem and Gray teach a spot test (Can. Inst. Food Sci. Technol. J. 14:150, 1981) that is a colorimetric test that monitors the free fatty acid content of the oil based on the pH of the oil. The diagnostic colors of the pH indicator are blue, green and yellow. Alkaline contaminant materials can also be detected in oils. Other calorimetric tests include those of U.S. Pat. Nos. 2,770,530; 3,030,190; 3,615,226; and 2,953,439.
Colorimetric tests can be problematic because the color indicators should be readily visualized from the background colors of the test. Colors such as yellow, light reds or light greens can be difficult to read because the degraded oils can also be colored.
One of the factors determining the cooking quality of used frying oils is the amount of breakdown products in the oil. The amount of polar compounds in oil is important because, for example, the presence of increased amounts of polar compounds detrimentally affect taste and oil viscosity. For example, increased amounts of polar compounds tend to produce a less viscous oil and this characteristic impacts the permeability of the oil into food, for example, in a flying apparatus. In general, reduced penetration of oils into foods is preferred for appearance, taste and health reasons.
A number of European countries have specific regulations for frying oils. Some of the European countries have regulations to restrict the amount of polar materials in oils. For example, Austria, Belgium, France, Germany, Hungary, Italy, Spain and Switzerland require, depending on the country, no more than between 2% 1 to 27% polar compounds in oils used with food. The German government has determined that the presence of 27% polar compounds corresponds to 0.7% of oxidized fatty acids insoluble in petroleum ether (Firestone, D. et al. “Regulation of Frying Fats and Oils” in Food Technology February 1991, pp 90–93), and that levels above this amount are not acceptable. It is likely that agencies of other governments will also institute regulations and quality assurance guidelines requiring the regular replacement of oil.
Many users of oxidative-sensitive and hydrolysis-sensitive oils employ routine periodic replacement programs to ensure that the oils maintain a useful taste and consistency. For a restaurant or a fast-food chain, it is difficult to evaluate the quality of the used cooking oil while on the restaurant premises other than by merely looking at its color, smelling the oil and/or observing the flying properties. Some restaurants with heavy fry demands and those with higher quality standards may discard and replace their oils after a relatively short time, irrespective of taste or consistency. A routine discard policy can be costly. A method to assess oil that is rapid and easy to use could save restaurants the time and money required to routinely and blindly replace their oil.
Thin layer chromatography (TLC) was originally developed as a method for separating lipids. TLC involves the use of a thin layer of adsorbent (e.g., silica gel) coated over a backing or solid surface such as glass, or the like. A sample to be analyzed is placed on the adsorbent and an edge of the thin layer is exposed to a solvent that travels up the thin layer, separating the compounds within the sample based on their relative affinities for the solvent and the adsorbent. The compounds can be identified by comparison of the separated sample with known standards.
In general, the technique has excellent resolving power and can be adapted to measure a variety of chemical constituents in a test sample. Where TLC is used to determine the amount of polar compounds in oil, a test sample is spotted onto a TLC plate and the plate is subjected to a solvent, such as petroleum ether, diethyl ether or glacial acetic acid. The solvent wicks up the adsorbent, toward the test sample. The test sample separates based on the relative affinities of the solvent and the test sample for the reactive groups on the adsorbent coating. The less polar compounds travel along the TLC plate faster than highly polar compounds. In this way TLC is used to detect the polar constituents in a test sample. TLC is a relatively sophisticated technique that is generally employed by scientists in a laboratory setting.
The American Oil Chemists' Society have standard methods for evaluating the quality of freshly refined cooking oil, including the amount of polar compounds in the oil. This method is published by the International Organization for Standardization and is provided as a publication ISO 8420:1990(E) entitled “Determination of Polar Compounds Content in Animal and Vegetable Fats and Oils” herein after referred to as “ISO 8420”). Other methods for quantifying the amount of polar compounds in oil include High Pressure Liquid Chromatography (HPLC), Liquid Chromatography (LC), Gas Chromatography (GC), and the like. ISO 8420 and other current procedures that quantify the amount of polar compounds in oil are complex and impractical for the cooking staff in an eating establishment.
The tests that are currently commercially available can have a number of problems. Many of the methods currently available for testing oil quality require expensive equipment that must be calibrated on a regular basis. These tests measure the polar compounds in oil indirectly. For example, pH changes are an indirect measure of the amount of polar compounds in an oil. A number of these tests require laboratory skill and periods of time greater than 2–3 hours.
Consequently, a long felt need exists for a quick reliably and easily performed test method to determine the amount of polar compounds in oil. A further need exists for a device to facilitate the performance of such a method.